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Light
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Light

Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. The three basic dimensions of light (and of all electromagnetic radiation) are brilliance (or amplitude), color (or frequency), and polarization (or angle of vibration). Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles.

Table of contents
1 Theories about light
2 Visible light wavelengths
3 The speed of light
4 Optics
5 Color and wavelengths
6 Measurement of light
7 Light sources
8 A light wave

Theories about light

Early Greek ideas

In
55 BC Lucretius, continuing the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles.

10th century optical theory

The scientist Abu Ali al-Hasan ibn al-Haytham (965-c.1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light. Alhazen's work did not become known in Europe until the late 16th century.

Particle theory

Pierre Gassendi, an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the 'plenum'. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.

Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.

The 'plenum'

Descartes held that light was a disturbance of the 'plenum', the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light travelled faster in a denser medium, by analogy with the behaviour of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.

Wave theory

In the 1660s Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the 'aether'. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye.

Another supporter of the wave theory was Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.

Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was later disproved.

Foucault's result in favour of the wave theory

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845 Faraday discovered that the angle of polarisation of a beam of light as it passed through a polarising material could be altered by a magnetic field. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether.

Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation. He first stated this in 1862 in On Physical Lines of Force. In 1873 he published Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. The technology of radio transmission was, and still is, based on this theory.

The constant speed of light predicted by Maxwell's equations contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. A solution to this contradiction would later be found by Albert Einstein.

Particle theory revisited

The wave theory was accepted until the late 19th century, when Albert Einstein described the photoelectric effect, by which light striking a surface caused elecrons to change their momentum, which indicated a particle-like nature of light. This clearly contradicted the wave theory, and for years physicists tried to rectify this contradiction without success.

Quantum theory

This theory, described by Max Planck in 1900, described light as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.

Wave-particle duality

This is the modern theory that explains the nature of light, and in fact of all particles. It was described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect, as well as Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature, and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

Visible light wavelengths

Visible light is that portion of the spectrum between the wavelengths of about 400 nanometers (abbreviated nm) and 800 nm (in air). Light can also be characterized by its frequency. The frequency and wavelength of light obey the relation :.

The speed of light

See speed of light. Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities (i.e. those associated with a direction). The speed of light is a scalar quantity (i.e. it has no direction), and therefore speed is the correct term.

Speed-of-light formula

,

where λ is the wavelength, f is the frequency, v is the speed of the light. If the light is travelling in a vacuum, then v = c, thus

,

where c is the speed of light. We can express v as

where n is a constant (the
refractive index) which is a property of the material through which the light is passing.

Changes in the speed of light

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second); however, when light passes through a transparent substance such as air, water or glass, its speed is reduced, and it suffers refraction. Thus, n=1 in a vacuum and n>1 in matter.

History of the measurement of the speed of light

The speed of light has been measured many times, by many physicists. The best early measurement is Olaus Roemer's (a Danish physicist), in 1676. He had developed a method for measuring light. He observed and noted the motions of Jupiter and one of its moonss with a telescope. It was possible to time the revolution of the moon because it was eclipsed by Jupiter at regular intervalss. Roemer discovered that the moon revolved around Jupiter once every 42-1/2 hours when Earth was closest to Jupiter. The problem was that when Earth and Jupiter were not as close, the moon's revolution seemed to be more. It was clear that light took longer to reach Earth when it was farther away from Jupiter. The speed of light was calculated by analyzing the distance between the two planets at various times. Roemer reached a speed of 227,000 kilometers per second (approximately 141,050 miles per second).

Albert A. Michelson improved on Roemer's work in 1926. He used rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 miles/second (299,796 kilometers/second). In daily use, the figures are rounded off to 186,000 mi/s and 300,000 km/s.

Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

Color and wavelengths

The different wavelengths are interpreted by the human brain as colors, ranging from red at the longest wavelengths (lowest frequencies) to violet at the shortest wavelengths (highest frequencies). The intervening frequencies are seen as orange, yellow, green, blue, and, conventionally, indigo. The frequencies of the spectrum immediately outside the range the human eye is able to perceive are called ultraviolet (UV) at the high frequency end and infrared (IR) at the low. Though humans cannot see IR, we do perceive it by receptors in the skin as heat. Cameras that can pick up IR and convert it to visible light are called night-vision cameras. UV radiation is not perceived by humans at all except in a very delayed fashion, as overexposure of the skin to UV light causes sunburn, or skin cancer. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR using pits in their heads.

Measurement of light

The following quantities and units are used to measure light.

See also: Photometry

Light sources

A light wave


The electric and magnetic fields are perpendicular to the direction of travel and to each other.


See also: Huygens' principle, Color temperature, Illumination, International Commission on Illumination, Wave-particle duality, Light pollution, photic sneeze reflex